With respect to using a membrane for concentration and separation of an aqueous solution of an organic substance, reverse osmosis has been put into practical use for concentration of some of the low-concentration aqueous organic substance solutions. During reverse osmosis a pressure higher than the osmotic pressure of a separation liquid must be applied to a liquid that is to be subjected to separation. However, reverse osmosis is inapplicable to high-concentration aqueous solutions of an organic substance because, the osmotic pressures are naturally high. Thus, the concentrations of separable solutions are limited when reverse osmosis is used.
By contrast, pervaporation and vapor permeation are new separation methods free from the influence of osmotic pressure. In pervaporation, the liquid to be subjected to separation is fed on the primary side of a membrane, while the pressure is decreased or a carrier gas is passed on the secondary side (the side of a permeated substance) of the membrane to permeate a substance to be separated in a gas state through the membrane. Vapor permeation is different from pervaporation in that a vapor from a mixture is fed on the primary side of a membrane. The substance permeated through the membrane can be recovered by cooling and condensing the permeated vapor.
Many research examples of pervaporation have been reported to date. For example, U.S. Pat. Nos. 3,750,735 and 4,067,805 disclose examples of organic substance/water separation with a polymer having active anionic groups. U.S. Pat. Nos. 2,953,502 and 3,035,060 disclose examples of ethanol/water separation with a cellulose acetate membrane and a polyvinyl alcohol membrane, respectively. In Japan as well, Japanese Patent Publication-A No. 109204/1984 discloses examples of use of a cellulose acetate membrane and a polyvinyl alcohol membrane. Japanese Patent Publication-A No. 55305/1984 discloses an example of use of a crosslinked polyethyleneimine membrane. Further, Japanese Patent Publication-B Nos. 10548/1979, 10549/1979 and 49041/1984 disclose experimental examples of separation of a water/organic substance mixture by using a synthetic polymer membrane having ionic groups introduced thereinto. However, the separation performances of the membranes disclosed in these publications exhibit low rates of permeation, thus leading to poor use of the membranes.
On the other hand, attempts to effect separation, based on a difference in size between molecules capable of permeation, by using a membrane having a small free volume have heretofore been made, the theory being that permeation of a liquid or a vapor is governed by dissolution and diffusion (see J. Memb. Sci., 30. (1987)).
However, there is a disadvantage with a membrane having a small free volume ad which has a low rate of permeation therethrough, even though the separation factor thereof is large. Thus, attempts to utilize a difference in solubility between substances have heretofore been made to increase the rate of permeation and the selectivity. For example, Japanese Patent Publication-A No. 24007/1981 discloses a permeable membrane of an aromatic polyamide-imide having sulfone groups introduced into the molecular chains thereof to thereby improve the rate of permeation of water. However, an increase in the amount of sulfone groups in the polymer correspondingly leads to poor membrane-forming properties, a decrease in the mechanical strength of the membrane obtained, and a loss in selectivity based on molecular sizes, due to swelling. The result is a loss of selectivity during separation of water and an organic substance from each other.
On the other hand, examples of membranes having excellent separation performance, such as membranes made of anionic polysaccharides and derivatives of polysaccharides, are disclosed in Japanese Patent Publication-A No. 129104/1985. These membranes, however, potentially involve such inevitable problems peculiar to natural polymers as depolymerization with an acid or an alkali and decomposition with bacteria, the result being that good durability, chemical resistance, etc., of the membranes cannot be expected. Further, membranes excellent in both the rate of permeation and the separation factor are disclosed in Japanese Patent Publication-A Nos. 182005/1988 and 11607/1989, in which the membranes are mainly made of a material having carboxyl groups.
Most of these exemplifying membranes require a technology for forming a thin membrane, such as a coating, to increase the rate of permeation when they are put into practical use. However, difficulties in the practical use of these membranes are encountered because of defects which are due to the presence of fine particles and other impurities, and problems of cracking of the coating layer and the like.